Separation techniques such as reverse osmosis, ultrafiltration and microfiltration are widely used today in industry. Many advantages have been realized by employing these techniques, among which are the reduction in time required for effecting separation, efficiency in separation, the use of mild operating conditions such as room temperature separations, the reduction in operating costs as compared to older techniques such as evaporation, chemical precipitation, and ultracentrifugation, and the capability to separate species previously considered inseparable. The present invention is particularly concerned with membrane separations by ultrafiltration techniques, although it can be applied to some of the other above-mentioned separation techniques.
Ultrafiltration is a separation process wherein a solution or suspension containing a solute, colloidal particle or suspended particle of greater dimensions than the solvent it is dissolved in, is fractionated by being subjected to such pressure as to force the solvent through a porous filter, particularly a polymeric membrane (see for example U.S. Pat. Nos.; 3,615,024; 3,526,588; 3,556,305; 3,541,005; and 3,549,016; all of which are hereby incorporated herein by reference to be generally illustrative of the types of polymeric membranes contemplated), although the filter can be of the nonpolymeric type such as ceramic. The membranes used in ultrafiltration may be of various configurations such as hollow fiber, flat sheet, spiral wound or tubular. Preferably, for the purposes of the present invention, hollow fiber polymeric membranes are employed.
Membrane separation systems are usually operated in a cross-flow mode whereby the process fluid flow (i.e. the "feed stream" to be separated) is tangential to the surface of the polymeric membrane. That is, the process fluid to be treated enters the separation module via the process fluid inlet, flows parallel to the surface of the membrane on the same side as the process fluid inlet and outlet are located, leaves the separation module via the process fluid outlet, and optionally, is recycled back to the separation module for further treatment. A portion of the process fluid passes through the membrane as permeate. This type of separation module may be used for various purposes such as; to concentrate a fluid, in which case the desired product is the fluid leaving the separator through the process fluid outlet; to purify a fluid, in which case the desired product can be the permeate or the fluid leaving the separator through the process fluid outlet; or to separate one or more components from a fluid, in which case the desired product may be the fluid passing through the membrane as permeate, the fluid leaving the separator through the process fluid outlet, the component(s) retained by the membrane, or combination thereof.
During use, the side of the membrane contacting the process fluid can become fouled by material retained by the membrane. Such fouled membranes can be cleaned for reuse by such techniques as; mechanical cleaning, the removal of foulant material by, for example, using a brush, rod or sponge; fast-flush, the pumping of fluid across the fouled surface of the membrane at high flow rate to physically dislodge and remove the foulant; fast-flush with reverse flow, the pumping of fluid across the fouled surface of the membrane at high flow rate with periodic reversal of the flow direction to physically dislodge and remove the foulant; chemical cleaning, the contacting of the fouled surface of the membrane with a chemical cleaning fluid; pressure backwash, the pumping of fluid, for example permeate or water, under pressure through the membrane from the permeate side to the process fluid side such that the fluid physically dislodges and removes foulant material from the surface of the membrane; or a combination of two or more of the above-mentioned techniques.
In the above-described techniques of fast-flush, chemical cleaning and pressure backwashing, pressure is usually created by means of a pump. This can give rise to hydraulic pressure surges that can damage the membrane. Therefore, it is important that the fluid pressure is carefully controlled so the pressure difference between the fluid on one side of the membrane and that on the other side of the membrane does not exceed the maximum allowable transmembrane pressure difference for that particular membrane. The maximum allowable transmembrane pressure difference for a particular membrane is the maximum pressure difference between opposite sides of a membrane that can be accommodated by the membrane without damage resulting.
Certain membrane configurations with narrow process fluid flow paths may become severely fouled, whereby the foulant restricts or even prevent the free flow of process fluid across the membrane surface. If this occurs, the fast-flush and chemical cleaning techniques may be insufficient to adequately clean the membrane, and mechanical cleaning and/or pressure backwashing may be required to achieve satisfactory cleaning. However, mechanical cleaning may be impractical for these membrane configurations because of their narrow process fluid flow paths, and pressure backwashing, as discussed above, has the disadvantages of; having to carefully control the pressure of the fluid in order to avoid damage to the membranes; and the addition of extra pumping capacity which adds to both the initial cost of the system and to the overall operating cost of the system.
The system design described in copending U.S. patent application Ser. No. 331,471 and the operation of this system described in copending U.S. patent application Ser. No. 331,476 (both commonly assigned to the same assignee as the present invention) presumably solves the problem of hydraulic pressure surges leading to damaged fibers. A suction backwash procedure is described in the above-mentioned patent applications to aid in the cleaning of the membranes. To activate the suction backwash procedure, a permeate pump is used to draw permeate through the hollow fibers by way of the process lines. While otherwise an improvement to the systems available heretofore, there are several problems and limitations to this design. First, the system described in U.S. patent application Ser. No. 331,471 is very complex, more costly than necessary and requires at least two pumps of equal size, one process pump, and one permeate pump. This additional pumping capacity not only adds cost to the initial system construction, but also increases the energy requirement during operation. Secondly, because the process fluid comes into contact with the permeate pump and associated permeate lines, the permeate lines and permeate pump become contaminated with the process fluid. This limits the possible applications of this design. After use of the suction backwash procedure and prior to operation in the standard mode, the system must be recleaned and/or re-sterilized.
An essential part of many separation techniques, as in membrane filtration, is the ability to keep the filtrate separate from the feed and therefore avoid mixing the permeate with the process fluid. This is beneficial in applications where the filtrate stream must remain sanitary, such as in food or pharmaceutical applications, or where the feed stream can contaminate the filtrate, such as in waste applications.